Method and apparatus for stacking electrical components using via to provide interconnection

ABSTRACT

An efficient chip stacking structure is described that includes a leadframe having two surfaces to each of which can be attached stacks of chips. A chip stack can be formed by placing a chip active surface on a back surface of another chip. Electrical connections between chips and leads on the leadframe are facilitated by bonding pads on chip active surfaces and by via that extend from the bonding pads through the chips to the back surfaces.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 10/987,468, filed Nov. 12, 2004, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to multi-chip stacking fabrication methods and, more particularly, to fabrication of thin packages containing multi-chips.

2. Description of Related Art

Personal devices that require a large number of electronic components to be provided in a small volume are rapidly proliferating. A pocket-sized personal music player that includes a hard disk is only one example of such a device. Today's personal electronic devices require that more and more functionality must be provided in a relatively small space. Traditionally, this functionality was provided by multi-chip electronic devices fabricated by placing chips on a two-dimensional substrate such as a printed circuit board (PCB). As circuit density increased, methods were devised for stacking multiple chips, thereby trading a scarce resource of substrate area for space in a third dimension. Several prior-art structures for stacking multiple chips have been devised, but none has proven to be wholly satisfactory. The need to stack components typically requires fabricating a superstructure that supports the stacked components. This superstructure adds to the volume and weight, and therefore to the cost, of the component stack, thereby offsetting an advantage that may be gained by stacking. Some stacking structures make efficient use of space, but tend to be complicated and expensive to fabricate. Less complicated and expensive stacking structures may either fail to make efficient use of space and/or present problems in disposing of the heat generated by chips in a stack. Other stacking structures include delicate wires that may introduce reliability concerns. Still other stacking structures may introduce reliability concerns at the level of PCB interconnection.

A need thus exists in the prior art for a stacking structure that is efficient in its use of space while being easy to fabricate. A further need exists for a structure that achieves reliable interconnection with a PCB.

SUMMARY OF THE INVENTION

The present invention addresses these needs by providing a chip stacking structure wherein chips have via that extend through the chip, thereby obviating the need for external wires to form electrical connections either between chips or with external leads. The invention herein disclosed comprises a leadframe having a plurality of leads disposed at a periphery of the leadframe. According to an exemplary embodiment, each lead has a lead inner portion and a lead outer portion. The lead outer portion may connect reliably with a substrate such as a printed circuit board (PCB). Each lead inner portion comprises a first surface and a second surface. The invention further may comprise a first chip stack formed of at least one chip, each chip having an active surface, a back surface, a plurality of first bonding pads disposed on the active surface, and a plurality of first chip via. Each first bonding pad has a bonding wall, and each first chip via has insulating material covering an inner wall of the first chip via. The insulating material does not cover a bonding wall. Each of the plurality of first chip via extends from a first bonding pad through the chip to the back surface. This embodiment further comprises a second chip stack formed in a manner similar to the formation of the first chip stack. Chips in the second chip stack comprise active surfaces, back surfaces, second bonding pads, and second chip via. The second chip via have insulating material covering inner walls thereof. The second bonding pads have bonding walls that are not covered by insulating material. This embodiment of the chip stacking structure may be formed by filling each first chip via with conducting material that electrically connects each first bonding pad to the first surface of a lead inner portion. Similarly, each second chip via may be filled with conducting material, electrically connecting each second bonding pad to the second surface of a lead inner portion.

Another embodiment of the present invention comprises a chip stacking structure having a plurality of chip stacks, each chip stack including at least one chip. Each chip comprises an active surface, a corresponding back surface, and a plurality of bonding pads disposed on the active surface. Each bonding pad has a bonding wall. Each chip further comprises a plurality of chip via having inner walls and extending from the plurality of bonding pads through the chips to the back surfaces. Insulating material covers the inner walls but does not cover the bonding walls. The chip stacking structure further comprises a leadframe having a plurality of leads disposed at a periphery thereof with the plurality of leads having lead inner portions and lead outer portions. The lead inner portions have first surfaces and second surfaces. A first chip stack is positioned with a first active surface facing the first surfaces and with a first plurality of bonding pads aligned with and making contact with the lead inner portions. A second chip stack is positioned with a second active surface facing the second surfaces and with a second plurality of bonding pads aligned with and making contact with the lead inner portions. Conductive material electrically connects bonding walls in the first chip stack to the first surfaces. Similarly, conductive material electrically connects bonding walls in the second chip stack to the second surfaces.

The present invention further comprises a method of stacking semiconductor chips. An implementation of the method comprises providing a leadframe having a plurality of leads disposed at a periphery thereof, the plurality of leads having lead inner portions and lead outer portions. The lead inner portions have first surfaces, second surfaces, and lead via that extend through the lead inner portions. An aspect of this implementation of the method comprises providing a first chip stack comprising at least one chip having an active surface, a back surface, and a plurality of bonding pads on the active surface. Each chip further comprises a plurality of chip via extending from the plurality of bonding pads through the chip to the back surface. Another aspect of the method positions the first chip stack with a first active surface facing the first surfaces and with a first plurality of bonding pads aligned with and making contact with the inner portions. A second chip stack also is provided, the second ship stack likewise comprising at least one chip having an active surface and a back surface. Each chip in the second chip stack also has a plurality of bonding pads on the active surface and a plurality of chip via that extend from the plurality of bonding pads through the chip to the back surface. The method further comprises positioning the second chip stack with a second active surface of the second chip stack facing the second surfaces such that a second plurality of bonding pads of the second chip stack is aligned with and makes contact with the lead inner portions.

While the apparatus and method has or will be described for the sake of grammatical fluidity with functional explanations, it is to be expressly understood that the claims, unless expressly formulated under 35 U.S.C. 112, are not to be construed as necessarily limited in any way by the construction of “means” or “steps” limitations, but are to be accorded the full scope of the meaning and equivalents of the definition provided by the claims under the judicial doctrine of equivalents, and in the case where the claims are expressly formulated under 35 U.S.C. 112 are to be accorded full statutory equivalents under 35 U.S.C. 112.

Any feature or combination of features described herein are included within the scope of the present invention provided that the features included in any such combination are not mutually inconsistent as will be apparent from the context, this specification, and the knowledge of one skilled in the art. For purposes of summarizing the present invention, certain aspects, advantages and novel features of the present invention are described herein. Of course, it is to be understood that not necessarily all such aspects, advantages or features will be embodied in any particular embodiment of the present invention. Additional advantages and aspects of the present invention are apparent in the following detailed description and claims that follow.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a plan view of a portion of a leadframe embodiment constructed according to the present invention;

FIG. 2 is a plan view of a chip that may connect to the leads of a leadframe of the type illustrated in FIG. 1;

FIG. 3 is a plan view of a partial stacked structure comprising a single chip connected to the leads of a leadframe;

FIG. 4 is a is a cross-sectional view, taken along the line 4-4′, of the chip/leadframe combination illustrated in FIG. 3;

FIG. 5A is a cross-sectional view of an embodiment of a chip comprising chip via and bonding pads according to the present invention;

FIG. 5B is a cross-sectional view showing details of a chip via and bonding pad illustrated in FIG. 5A.

FIG. 6 is a cross-sectional view of an embodiment of two chips connected to a leadframe according to the present invention;

FIGS. 7A-7C are cross-sectional views of embodiments of a stacked structure comprising two chips configured according to the present invention;

FIGS. 7D and 7E are cross-sectional views of modified embodiments of a stacked structure comprising two chips not having coaxially aligned chip via;

FIG. 8 is a cross-sectional view of an embodiment of four chips connected to a leadframe according to the present invention;

FIGS. 9A-9C are cross-sectional views of implementations of a stacked structure comprising four chips arranged according to the present invention;

FIGS. 9D and E are cross-sectional views of modified embodiments of a stacked structure comprising two pairs of chips, the chip via of one pair not being coaxially aligned with chip via of the other pair of chips;

FIG. 9F is a cross-sectional view of a stacked chip structure attached to one side of a lead frame according to the present invention;

FIG. 10 is a plan view of an embodiment of a leadframe comprising a supporting pad according to the present invention;

FIG. 11 is a cross-sectional view of an embodiment of two chips connected to a leadframe that comprises a supporting pad according to the present invention;

FIGS. 12A-12C are cross-sectional views of embodiments of a stacked structure comprising four chips combined with a leadframe having a supporting pad according to the present invention;

FIG. 13 is a plan view of a four-chip stacked structure formed on a leadframe having a supporting pad according to the present invention;

FIGS. 14A-14C are cross-sectional views of stacked structures comprising four chips in an embodiment comprising a leadframe having a supporting pad;

FIGS. 14D and 14E are cross-sectional views of single-sided stacked structures fabricated according to the present invention; and

FIG. 15 is a flow diagram that describes an implementation of a method of forming a stacked chip structure according to the present invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

Reference will now be made in detail to the presently preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same or similar reference numbers are used in the drawings and the description to refer to the same or like parts. It should be noted that the drawings are in simplified form and are not to precise scale. In reference to the disclosure herein, for purposes of convenience and clarity only, directional terms, such as, top, bottom, left, right, up, down, over, above, below, beneath, rear, and front, are used with respect to the accompanying drawings. Such directional terms should not be construed to limit the scope of the invention in any manner.

Although the disclosure herein refers to certain illustrated embodiments, it is to be understood that these embodiments are presented by way of example and not by way of limitation. The intent of the following detailed description, although discussing exemplary embodiments, is to be construed to cover all modifications, alternatives, and equivalents of the embodiments as may fall within the spirit and scope of the invention as defined by the appended claims. It is to be understood and appreciated that the process steps and structures described herein do not cover a complete process flow for the manufacture of stacking structures. The present invention may be practiced in conjunction with various integrated circuit fabrication techniques that are conventionally used in the art, and only so much of the commonly practiced process steps are included herein as are necessary to provide an understanding of the present invention.

Referring more particularly to the drawings, FIG. 1 is a plan view of a portion of a leadframe embodiment constructed according to the present invention. The portion of the leadframe illustrated in FIG. 1 comprises a plurality of leads having lead inner portions 130. Each lead inner portion 130 may have a lead via 140 formed therein. The lead inner portions 130 are extended to reach an active surface of a chip that may be positioned to make contact with the lead inner portions 130. The lead inner portions 130 may be configured without lead via 140 in modified embodiments.

FIG. 2 is a plan view of a chip 201 a that may connect to the leads of a leadframe of the type illustrated in FIG. 1. Chip 201 a has an active surface 211 a and a corresponding back surface 221 a (FIG. 4, infra). A plurality of first bonding pads 231 a is disposed on the active surface 211 a of the chip 201 a. Each first bonding pad 231 a has associated with it a first chip via 241 a that extends from the first bonding pad 231 a through the active surface 211 a of the chip 201 a to the back surface 221 a (FIG. 4) of the chip 201 a. According to a typical embodiment, the first bonding pads 231 a and their associated first chip via 241 a are fabricated to align with lead inner portions 130, which may comprise lead via 140 as illustrated in FIG. 1.

FIG. 3 is a plan view of a partial stacked structure comprising a single chip connected to the leads of a leadframe. The embodiment illustrated in FIG. 3 comprises a leadframe portion as illustrated in FIG. 1 placed in contact with the chip 201 a illustrated in FIG. 2. Lead via 140 in the lead inner portions 130 are coaxially aligned with first chip via 241 a (FIG. 2). The coaxial alignment of the first chip via 241 a with the lead via 140 assures that the lead inner portions 130 also align with first bonding pads 231 a on the active surface 211 a of the chip 201 a. The alignment of first bonding pads 231 a with the lead inner portions 130 assures that first bonding pads 231 a are able to establish electrical contact with the lead inner portions 130.

FIG. 4 is a cross-sectional view taken along the line 4-4′ of the chip/leadframe combination illustrated in FIG. 3. This view illustrates the active surface 211 a and the back surface 221 a of the chip 201 a. First bonding pads 231 a formed on the active surface 211 a connect to first chip via 241 a that extend from first bonding pads 231 a through the chip 201 a to the back surface 221 a of the chip 201 a. The lead via 140 are coaxially aligned with first chip via 241 a. Each lead inner portion 130 illustrated in FIG. 4 comprises a first surface 110 and a second surface 120. In the illustrated embodiment, the active surface 211 a of chip 201 a is secured to the first surfaces 110 of the lead inner portions 130 that form a portion of the leadframe.

An electrically conductive material such as solder may be used to fill the lead via 140 and first chip via 241 a. Solder may flow over first bonding pads 231 a thereby providing mechanical as well as electrical connection of the lead inner portions 130 to first bonding pads 231 a. According to another embodiment (not illustrated), chip 201 a has no via, and the first surface 110 of the lead inner portions 130 is secured to the active surface 211 a of chip 201 a by a solid or liquid adhesive.

FIG. 5A is a cross-sectional view of an embodiment of a chip 201 a comprising first chip via 241 a and first bonding pads 231 a according to the present invention. Although a chip may comprise many such first chip via 241 a and first bonding pads 231 a, only two of each are shown in FIG. 5A for clarity. A single first chip via 241 a and its associated first bonding pad 231 a are selected and designated as 240 a for further discussion below.

FIG. 5B is a cross-sectional view of the selected first chip via 241 a and first bonding pad 231 a designated as 240 a in FIG. 5A. The first bonding pad 231 a has a bonding wall 230 a. As shown in FIG. 5B, the first chip via 241 a may include an insulated coating 239 a on an inner wall of the first chip via 241 a. This insulated coating 239 a, which does not extend to cover the bonding wall 230 a of the first bonding pad 231 a, can prevent electrical contact from occurring between conducting material that may be placed within the first chip via 241 a and active areas internal to the chip 201 a (FIG. 5A). Methods are known in the art for forming the insulated coating on the inner wall of the first chip via 241 a. For example, the first chip via 241 a may be formed by a burning operation performed with a laser. The laser, in burning the first chip via 241 a, may do so with a temperature high enough to oxidize semiconductor material that forms the chip 201 a. If the chip 201 a is formed of silicon, then the oxidized semiconductor material is silicon dioxide, known to be insulating material. Although not specifically illustrated, the via illustrated in FIGS. 6, 7A-7E, 8, 9A-9F, 11, 12A-12C, 13, and 14A-14E may include insulated coatings on inner walls in a manner represented in FIG. 5B.

FIG. 6 is a cross-sectional view of an embodiment of two chips connected to a leadframe according to the present invention. The lower portion of the diagram duplicates FIG. 4 wherein chip 201 a is joined with the first surfaces 110 of the lead inner portions 130 on the leadframe. In FIG. 6 another chip 202 a is added to the combination. Chip 202 a has an active surface 212 a and a back surface 222 a. The active surface 212 a has formed thereon second bonding pads 232 a, which connect to second chip via 242 a that extend through the chip 202 a to the back surface 222 a. The second chip via 242 a can be formed with an insulated coating as described above with reference to FIG. 5B. Chip 202 a aligns with chip 201 a and with the lead inner portions 130 so that first chip via 241 a, second chip via 242 a, and lead via 140 are coaxially aligned. The alignment assures that second bonding pads 232 a make electrical contact with the second surfaces 120 of the lead inner portions 130.

In modified embodiments configured without lead via 140, alternative methods may be employed to create electrical contact between, for example, first surfaces 110 and first bonding pads 231 a. For example, a coating of conductive material (e.g., solder) may be provided on first surfaces 110 to enhance adhesion of a first chip via 241 a to a first surface 110. Moreover, convex conductive features (e.g., “bumps”) may be formed on first surfaces 110 in order to enhance alignment and adhesion of first chip via 241 a to first surfaces 110.

FIG. 7A is a cross-sectional view illustrating an embodiment of a two-chip stacked structure fabricated according to the present invention. Generally, this embodiment is configured as illustrated in FIG. 6 except that leads 100 are shown as well. Each lead 100 comprises an lead inner portion 130 as already described and an outer portion 135. The lead inner portion 130 may comprise lead via 140 that are coaxially aligned with first chip via 241 a and second chip via 242 a. The outer portions 135 may serve to provide a reliable electrical and mechanical connection to a substrate such as a printed circuit board (PCB). The illustrated embodiment exposes the respective back surfaces 221 a and 222 a of chips 201 a and 202 a, respectively, thereby enhancing dissipation of heat generated by the chips 201 a and 202 a. It should be noted that the embodiment illustrated in FIG. 7A has no active surface or wiring exposed. Therefore, this embodiment has no need of any encapsulation to protect components of the structure. The absence of encapsulation may promote increased thermal dispersion of heat generated by chips 201 a and 202 a.

FIG. 7B is a cross-sectional view of another embodiment of a two-chip stacked structure. This embodiment is similar to the embodiment illustrated in FIG. 7A except that parts chips 201 a and 202 a and parts of the leadframe are at least partially encompassed in an enclosure 400. A portion of a region 410 internal to the enclosure 400 may be filled with, for example, a molding resin, encapsulating the chips 201 a and 202 a and lead inner portions. The enclosure 400 in this embodiment encloses the lead inner portions 130 (FIG. 7A), but does not enclose the respective back surfaces 221 a and 222 a of chips 201 a and 202 a, respectively. Thermal dispersion of heat generated by the chips 201 a and 202 a is enhanced by the exposure of the back surfaces 221 a and 222 a.

FIG. 7C is a cross-sectional view of yet another embodiment of a two-chip stacked structure. This embodiment is similar to FIG. 7B except that an enclosure 500 surrounds all surfaces of the chips 201 a and 202 a. A region 510 is formed by the enclosure, wherein a material such as molding resin may occupy a portion of the region 510, encapsulating the chips 201 a and 202 a and lead inner portions.

FIGS. 7D and 7E illustrate examples of modified embodiments of the two-chip structures illustrated in, respectively, FIGS. 7A and 7B, wherein first chip via 241 a are not coaxially aligned with second chip via 242 a. The lead inner portions 130 in the embodiment illustrated in FIG. 7D comprise first lead via 141 and second lead via 142. First lead via 141 coaxially align with first chip via 241 a; second lead via 142 coaxially align with second chip via 242 a. The embodiment illustrated in FIG. 7E is configured in a manner similar to the example shown in FIG. 7D except for the inclusion of an enclosure 400 defining an internal region 410.

FIG. 8 is a cross-sectional view of an embodiment of four chips connected to a leadframe in accordance with the present invention. This embodiment is similar to the embodiment illustrated in FIG. 6 with the addition of chips 201 b and 202 b. Chip 201 b has an active surface 211 b and a corresponding back surface 221 b. A plurality of bonding pads 231 b is disposed on the active surface 211 b. Each bonding pad 231 b has associated with it a chip via 241 b that extends from the bonding pad 231 b to the back surface 221 b of the chip 201 b. Chip 201 b is stacked under chip 201 a with the active surface 211 b of chip 201 b facing and establishing contact with the back surface of chip 201 a.

The placement of chip 202 b relative to chip 202 a is similar to the placement of chip 201 b relative to chip 202 a. Chip 202 b has an active surface 212 b and a corresponding back surface 222 b. A plurality of bonding pads 232 b are disposed on the active surface 212 b, each bonding pad 232 b being connected to a chip via 242 b. The chip via 242 b extend from the bonding pads 232 b to the back surface 222 b. The active surface 212 b of chip 202 b faces and contacts the back surface 222 a (FIG. 6) of chip 202 a. Corresponding chip via 241 b, 242 b, and (see FIG. 6) 241 a and 242 a as well as lead via 140 are coaxially aligned, thereby facilitating electrical contact among the chips 201 a, 201 b, 202 a, and 202 b and the lead inner portions 130. In particular, an electrically conductive material such as solder may be used to fill corresponding chip via 241 b, 242 b, and (see FIG. 6) 241 a, and 242 a, thereby providing electrical connection among the bonding pads 231 b, 232 b, and (see FIG. 6) 231 a and 232 a. Solder may flow in the lead via 140 (FIG. 6) between first bonding pads 23 la and second bonding pads 232 a, thereby providing electrical connection as well to the lead inner portions 130. Accordingly, corresponding bonding pads 231 b, 231 a, 232 a, and 232 b may be electrically connected to lead inner portions 130 of corresponding leads.

It should be clear from the examples presented herein that the direction in which chips face, i.e. up or down, is not constrained by present description of the invention. Rather, the facing direction of chips can be chosen according to aspects of a particular design or application.

The structure illustrated in FIG. 8 may be extended by providing additional chips having active surfaces, back surfaces, bonding pads, and chip via of the type already described. For example, another chip could be added to the structure by placing the active surface of the additional chip to face either the back surface 221 b of chip 201 b or the back surface 222 b of 202 b. Limits to the process of stacking additional chips, if any, may be imposed, for example, by external space considerations.

FIG. 9A is a cross-sectional view of an embodiment of a four-chip stacked structure configured according to the present invention. The embodiment in FIG. 9A should be compared with the embodiment in FIG. 7A, the substantial difference between the two embodiments being the addition of chips 201 b and 202 b to the structure illustrated in FIG. 7A. Again, the exposure of back surfaces 221 b and 222 b in FIG. 9A enhances the ability of the structure to disperse heat generated by the chips. Electrical connection among chips and the leadframe inner leads 110 may be achieved, according to another representative embodiment, by means of conducting material (such as solder) that makes contact with a bonding wall 230 a (FIG. 5B). The conducting material further may fill via (e.g., first chip via 241 a illustrated in FIG. 5B and chip via 241 b illustrated in FIG. 8) and may make electrical contact with inner leads, e.g. inner lead portions 130 illustrated in FIG. 9A.

FIG. 9B is a cross-sectional view of a stacked structure embodiment comprising four chips in accordance with the present invention. This embodiment relates to the embodiment illustrated in FIG. 9A by the addition of an enclosure 600 that encloses the chips 201 b, 201 a, 202 a, and 202 b and the lead inner portions 130 (FIG. 9A). Molding resin may partially fill an internal region 710 formed by the enclosure. The molding resin may encapsulate the chips 201 b, 201 a, 202 a, and 202 b and lead inner portions. Back surfaces 221 b and 222 b are exposed in this embodiment, to dissipate heat generated by the chips 201 b, 201 a, 202 a, and 202 b more efficiently.

FIG. 9C is a cross-sectional view of another implementation of a four-chip stacked structure. In the present embodiment, all surfaces of chips 201 b, 201 a, 202 a, and 202 b are surrounded by an enclosure 700, thereby forming a region 710. As before, molding resin may occupy the region 710 and encapsulate the chips 201 b, 201 a, 202 a, and 202 b and lead inner portions.

FIGS. 9D-9F depict embodiments of other examples of four-chip structures fabricated according to the present invention. The embodiments illustrated in FIGS. 9D and 9E generalize the structures illustrated in respective FIGS. 9A and 9B to cases where first chip via are not coaxially aligned with second chip via. FIGS. 9D and 9E are not described in detail because of their similarity to FIGS. 7D and 7E. FIG. 9F describes another modified embodiment of a four-chip stacking structure fabricated according to the present invention. The four chips 204 a-204 d illustrated in FIG. 9F are stacked on a single side of the lead inner portions 130 and are connected electrically to the second surfaces 120 of the leads.

FIG. 10 is a plan view of an embodiment of a leadframe comprising a supporting pad 300 according to the present invention. The supporting pad 300 in the illustrated embodiment comprises at least one elongated chip supporting bar 310. (Four elongated chip supporting bars 310 are illustrated in FIG. 10.) The leadframe further comprises leads having lead inner portions 130 and lead via 140 as described above with reference to FIG. 1.

FIG. 11 is a cross-sectional view of an embodiment of two chips 201 a and 202 a connected to a leadframe that comprises a supporting pad 300 according to the present invention. The illustrated embodiment is similar to the embodiment described above with reference to FIG. 6, but differs by the provision of the supporting pad 300. The active surfaces 211 a and 212 a of chips 201 a and 202 a may be secured to the supporting pad 300 in a manner such that the supporting pad 300 does not interfere with first bonding pads 231 a and second bonding pads 232 a. In typical embodiments, the active surfaces 211 a and 212 a are secured to opposite surfaces of the supporting pad 300 by a non-conducting adhesive. The non-conductive adhesive may be either a solid or a liquid.

FIGS. 12A-12C, 13, and 14A-14D are cross-sectional diagrams illustrating embodiments of various forms of stacked structures that include a supporting pad 300. These embodiments are similar to the embodiments described with reference to respective FIGS. 7A-7C, 8, 9A-9C, and 9F except for the inclusion of a supporting pad 300 as described above with reference to FIGS. 10 and 11. FIG. 14E is a modified configuration of FIG. 14D, wherein an upper enclosure 1000 and a lower enclosure 1005 define an upper internal region 1010 and a lower internal region 1015, respectively.

FIG. 15 is a flow diagram that describes an implementation of a method of forming a stacked chip structure according to the present invention. According to the illustrated implementation, a leadframe is provided at step 1200. An exemplary embodiment of a portion of such a leadframe is illustrated in FIG. 1. The leadframe typically comprises a collection of leads having lead inner portions 130 and may comprise lead via 140 as already described. The lead inner portions 130 have first and second surfaces 110 and 120 as illustrated in FIG. 4. A first chip stack, having bonding pads and first chip via, is provided at step 1210, the first chip stack comprising at least one chip 201 a as illustrated in FIG. 4. As another example, a two-chip stack comprising chips 201 a and 201 b is illustrated in FIG. 8. The first chip stack is positioned on the leadframe at step 1220. Referring to FIG. 4 as an example, the chip stack may be positioned with first chip via 241 a aligned with lead via 140. The chip stack is placed such that first bonding pads 231 a on the active surface 211 a of the chip 201 a make contact with first surfaces 110 of the lead inner portions 130. The alignment of first chip via 241 a and lead via 140 assures that first bonding pads 231 a are aligned with and make electrical contact with the lead inner portions 130. A second chip stack, likewise having bonding pads and chip via, is provided at step 1230. As illustrated in FIG. 6, the second chip stack may comprise at least one chip 202 a. The second chip stack is positioned on the leadframe at step 1240. As with the placement of the first chip stack, the second chip stack is positioned such that second bonding pads 232 a on active surface 212 a of chip 202 a contact second surfaces 120 of the lead inner portions 130. Second chip via 242 a are aligned with lead via 140. This arrangement provides electrical contact between second bonding pads 232 a and the lead inner portions 130.

In view of the foregoing, it will be understood by those skilled in the art that the methods of the present invention can facilitate formation of efficient stacking structures for integrated circuits. The above-described embodiments have been provided by way of example, and the present invention is not limited to these examples. Multiple variations and modification to the disclosed embodiments will occur, to the extent not mutually exclusive, to those skilled in the art upon consideration of the foregoing description. For example, the embodiments illustrated in FIGS. 3, 4, 6, 7A-7E, 8, 9A-9F, 11, 12A-12C, 13, and 14A-14D illustrate chips and chip stack sets having an active surface that faces the leadframe inner lead surface. Other embodiments may comprise chips or chip stacks having one or more back surfaces that face the leadframe inner lead surface. Additionally, other combinations, omissions, substitutions and modifications will be apparent to the skilled artisan in view of the disclosure herein. Accordingly, the present invention is not intended to be limited by the disclosed embodiments, but is to be defined by reference to the appended claims. 

1. A method of stacking semiconductor chips, comprising: providing a leadframe having a plurality of leads disposed at a periphery thereof, the plurality of leads having lead inner portions and lead outer portions, the lead inner portions having first surfaces, second surfaces, and lead via extending through the lead inner portions; providing a first chip stack comprising at least one chip having an active surface, a back surface, a plurality of bonding pads on the active surface, and a plurality of chip via extending from the plurality of bonding pads through the chip to the back surface; positioning the first chip stack with a first active surface facing the first surfaces and with a first plurality of bonding pads aligned with and making contact with the lead inner portions; providing a second chip stack comprising at least one chip having an active surface, a back surface, a plurality of bonding pads on the active surface, and a plurality of chip via extending from the plurality of bonding pads through the chip to the back surface; and positioning the second chip stack with a second active surface facing the second surfaces and with a second plurality of bonding pads aligned with and making contact with the plurality of lead inner portions.
 2. The method as set forth in claim 1, further comprising disposing an insulated coating on inside walls of the plurality of chip via.
 3. The method as set forth in claim 1, wherein providing a first chip stack comprises: providing a first chip and a second chip, the first and second chips being joined such that the active surface of the second chip faces the back surface of the first chip; and coaxially aligning the plurality of chip via of the first and second chips with the lead via.
 4. The method as set forth in claim 3, further comprising electrically connecting the plurality of bonding pads of chips that form a stack.
 5. The method as set forth in claim 4, wherein electrically connecting the plurality of bonding pads comprises disposing conductive material in the plurality of chip via, the conductive material being electrically connected to the plurality of bonding pads.
 6. The method as set forth in claim 5, further comprising electrically connecting the plurality of bonding pads to the lead inner portions.
 7. The method as set forth in claim 1, wherein providing a second chip stack comprises: providing a third chip and a fourth chip, the third and fourth chips being joined such that the active surface of the fourth chip faces the back surface of the third chip; and coaxially aligning the plurality of chip via of the third and fourth chips with the lead via. 